1. Field of the Invention
The present invention relates to a photo-plating process which enables formation of a thick coating containing at least one of copper, nickel, cobalt, and tin in an electroless plating solution, the coating being tightly adhered only to the light-irradiated portions of a dye layer formed on a substrate. The present invention also relates to an article having a coating formed thereon by photo-plating. Accordingly, the present invention is useful for, e.g., forming electric circuit patterns on a printed circuit board.
2. Description of the Related Art
There is known a photo-plating process which takes advantage of a radiant energy of light to selectively deposit a metal only on the light-irradiated portions of a substrate. More specifically, in Japanese Laid-Open Patent Publication No. 205388/1990 and in Japanese Patent Publication No. 2950/1990 is disclosed a process which comprises using a photo-oxidation-reduction reaction of a semiconductor on absorbing light, thereby depositing a metal selectively only on the light-irradiated portions of a substrate having a semiconductor provided thereon. In this process, an aqueous metal-ion solution containing a reducing agent such as an alcohol and formaldehyde is used as the plating solution. Referring in more detail to the known process, the mechanism of the reaction for depositing a metal on the substrate is believed as follows. On irradiating ultraviolet (UV) light to a semiconductor such as TiO.sub.2 provided on a substrate and being immersed in an aqueous solution containing metal ions and a reducing agent, electrons which generate in the conduction band of the semiconductor reduce metal ions in the aqueous solution to form metal deposits on the semiconductor. The reducing agent supplies the electrons to the holes formed by the UV radiation in the valence band of the semiconductor, thereby preventing the holes from coupling with the electrons in the conduction band. The processes using conventional solutions, however, are disadvantageous in that the metal deposition to a thickness of several thousand angstroms or more on the surface of the semiconductor prevents the irradiated light from reaching the semiconductor, thereby ceasing further deposition of the metal. Thus a thick metal coating could not be obtained.
Another drawback is the poor adhesion strength of the film to the semiconductor, because the sites for initiating the reaction were limited to coarse semiconductor particles on the substrate.